Controlled coagulation of salt-spun polyacrylonitrile



G. W. STANTON ET AL April so, 1957 CONTROLLED COAGULATION OF' SALT-SPUNPOLYACRYLONITRILE Filed Jan. 2'?, 1954 s sheets-sheet 1 INV E NTORS.

April 30, 1957 G. w. STANTON ETAL y 2,790,700

CONTROLLED COAGULATION OF SALT-SPUN POLYACRYLONITRILE Filed Jan. 27.1954 n 3 Sheets-Sheet 2 l5 20 Z5 30 35 40 5y Way/Hf ZnC/Z /n CaagPressure Droplo. s.

ATTORNEYS.

April 30, 1957 G. w. STANTON ET Al.

CONTROLLED COAGULATION OF SALT-SPUN POLYACRYLONITRILE Filed Jan. 27,1954 INVENTORS George W. .S/an for) Theodore 5. Le/f'eAa//nk Thomas C.Spence' OTE t@ Conga/offen BY l ATTORNEYS.

United States Patent C firice CNTRLLED CAGULATION F SALT-SPUNPOLYACRYLONITRILE George W. Stanton, Walnut Creek, and Theodore B.

Lefferdink and Thomas C. Spence, Concord, Calif., assi'gnors to The DowChemical Company, Midland, Mich., a corporation of Delaware ApplicationJanuary Z7, 1954, Serial No. 406,516

5 Claims. (Ci. 18-5l) This invention relates to the Wet .spinning ofpolyacrylonitrile, and of linear fiber-forming copolymers containing atleast 85 percent of acrylon'itrile in the polymer molecule, from aqueoussaline solutions thereof, and is concerned particularly with .a methodfor controlling the rate of coagulation of the spun product so as toassure the production of fibers of consistency high quality.`

After the discovery in Germany by Dr. Herbert Rein (See U. S. 2,140,921)that many high polymers, including polyacrylonitri'le, can be dissolvedin and spun from concentrated aqueous solutions of certain highlyhydrated metal salts, such as zinc chloride, it has been reported by.other investigators that such salt .solutions commonly yield poorlibers when wet spun. The product 'has been described as brittle, opaqueand full of voids. ln consequence, much attention has been given to thediscovery of organic solvents .suitable for spinning polyacrylonitrileby either the wet or the dry spinning procedures.

The use of organic solvents, however, 'is expensive and results in atiber having an irregular cross-section, whereas the :fibers wet `spunfrom saline solutions are cylindrical. This difference between .fibersspun .from organic solvents and those spun from aqueous inorganic saltsis due to the different behaviors of the two types of solution when`brought into contact with a .setting medium. Dry spun organic solutionsof the polymer merely lose solvent to the evaporative atmosphere, andthe fiberforming material collapses on itself as fthe solvent isremoved. Wet spun organic solutions .of the polymer are converted intofibers by extraction of 'the organic medium from the spun dope by theaction of a nonsolvent in the coagulating bath. The material inthecoagul-ating bath which is a non-solvent for the polymer must bemiscible, however, with the solvent lfor the polymer. In common practicethe coagulant is water, and the polymer solvent is water-misci-ble. Whenspun from such organic solutions, the polymer Vremains vhydrophobic.Hence, the freshly coagulated fiber skin acts -as a semi-permeablemembrane in transferring solvent from the fiber to the bath and allowingessentially no migration of water into the core of the liber, even whenlthe bath contains a high concentration of solvent. ln consequence, thecores of the wet spun Jbers from organic solutions Ishrink and set dueto the removal of solvent therefrom land to the resultant radialcollapse Iof the .fiber to a -denser condition, forming a [fiber ofirregular cross-section. The effect, then, of both wet and dry spinningof organic solutions of hydrophobic -polymers, is primarily one ofremoving .solvent until the yconcentration of the polymer exceeds thecapacity of the lremaining solvent -to dissolve it. There is noformation of a stretchable gel, and the polymer can only be ren-Patented Apr. 30, 1957 dered plastic by heat in order to be stretchedsignificantly. Because the organic solutions behave in this way, thefreshly rspun bers cannot be subjected to any significant amount of coldstretching. Consequently, `to make ltine bers from organic solutions ofthe polymer, one needs spinnerets with very small orilices, and thisnecessitates high extrusion pressure to move the viscose solutionsthrough the spinneret-s.

The present inventors and cro-workers have disclosed in U. S. Patents2,648,646 and 2, 648,647 (which were copending with the parent of thepresent appli-cation) that it is possible to obtain good fibers of highacrlonitrile polymers by wet spinning aqueous saline solutions thereofin which the saline constituents are mixed salts, one of which ifscapable at higher concentrations of dissolving the polymer and the otherof which -is incapable of dissolving the polymer at yany concentrationin water. It is shown in lthose patents that the minimum concentrationof saline constituents in water to dissolve high acrylonitrile polymersis 55 percent.

It has been taught by Cresswell and his co-workers, in U. S. Patents2,558,730-2,558,735, inclusive, and 2,558,781, that useful fibers ofpolyaciylonitrile .may be wet `spun from aqueous ysaline solutionsthereof, provided that the coagulating bath is at a temperature not toeX- ceed -i- 10 C. and provided further that the bath be mainly water,with specific mention being made of water alone, aqueous ethanol (toprevent freezing the bath at temperatures down to -'l5 C), 0.24 normalhydrochloric acid, and dilute salt (sodium chloride) solutions. rIlheso-.coagu'lated fiber is described as a swollen or gelled article havingmarked elasticity and toughness. It can -be washed free of salt and hotstretched to make strong libers. The same patents state that when thesaline solutions ,of the polymer are coagulated at temperaturesmaterially above .-ll0" C., the product 'is a non-transparent, weak gel-having little or no ductility. Because of the high viscosity of thesaline solution of polymer, the solution is supplied under pressure tothe spinneret and its mobility is increased by heating it before itpasses lthrough .the small spinneret orifices into Athe cold coagu--lating bath. The oriiices `mentioned 'in U. S. 2,558,730 ,are of 55 to9.0 microns diameter, or from about 0.002 to 0.004 inch. A 40-iber tow,when given a standard after-treatment, had a denier of 97 (about 2.5 perfil) and a tenacity of 4.1 grams per denier, both being in a very usefulrange, though the 14 percent elongation indicates the toughness is notas great as may be desired .for many uses.

Each of the aforementioned patents relative to wet spinning of salinesolutions of -polyacryl'om'trile is an `advance over the prior art, yetnone 4of them 4teaches a spinning method which is free from certainserious limitations. Thus, as Ashown by the small orifices required tomake ne bers, and the low temperatures lrequired for .coagulation inwat-er and dilute acids, .alcohols or salts, it is apparent that ylowcoagulation 4temperature alone does not assure high rates of productionat low extrusion pressures. lt has been found, as well, that when bersare spun from the aforementioned mixed salt brines into such an abruptcoagulant as water, there is Aobtained a physically non-uniform product.This is due, apparently, to the initial formation of a hard sheath aboutthe still fluid core of the freshly spun ber, and any stretching toeffect a reduction in diameter results in rupturing'the-tber.

It is accordingly among the objects of this invention to provide amethod for wet spinning aqueous saline solutions of polyacrylonitrile,or of high acrylonitrile copolymers having similar characteristics,whether those solutions contain a single salt or a plurality of salts,whereby controlled coagulation may be obtained consistently: (a) to giveclear gels at any desired operating temperature up to 30 C.; (b) to.permit the freshly extruded fiber to be cold stretched `to effect areduction in diameter to a desired size smaller than the extrusionorifice; (c) to deliver the fibers in a uniformly gelled state to thewashing and hot stretching zones which normally follow coagulation; and(d) to effect substantial economy in the recovery of the salineconstituent of the spinning dope. A related object is to provide amethod whereby there may be produced consistently fibers which, at anydesired denier and tenacity, are capable of high elongation. Otherobjects may appear hereinafter.

It has now been found that markedly improved fibers may be spun fromaqueous salt solutions of polyacrylonitrile, or of high acrylonitrilelinear copolymers, containing a single salt or a mixture of solvent andnon-solvent salts, when the saline solution medium is extractedgradually and under controlled conditions in the coagulating bath.According to the invention, the saline solution of the polymer is spunfirst into an aqueous coagulant bath consisting of a solution in waterof the same salt or salts as are in the spinning dope, at concentrationscontrolled within a narrow range but whose absolute values vary withvariations in spinning rate, spinneret size, spinning dopeconcentration, and coagulation temperature. In all cases, thecoagulating -bath has a concentration of the salt or salts such that thespun fiber can be taken away from the spinneret orifice, withoutbreak-ing, at a linear rate 4 or more times that at which the spinningdope enters the bath from -the spinneret. Unless the freshly spun fiberis `coagulated under conditions which would permit such a takeaway rate,no significant dimensional adjustment can be made in the coagulatingbath,'and, for fine fibers, small orifices are required. Further, unlessthe fibers initially exhibit liquid-like flow, coagulation isnon-uniform, and lany after treatment gives non-uniform products. In thecase of zinc chloride solutions of polyacrylonitrile, to get thiseffect, the concentration in the coagulating bath must be at leastpercent and not to exceed -about 47 percent by weight, and for anyspecific set of operating conditions the optimum operative range ofconcentrations falls within a narrow bracket Within the stated range, aswill be illustra-ted hereinafter. When the strongly saline solution ofthe polymer lis spun into the appropriate lower concentration of thesame salt system, complete coagulation is not instantly effected and thefibers are not self-supporting for `a short distance, usually more thanl inch, from the spinneret, as will be described more fully.

Between lthe orifice and the point at which the fibers areself-supporting or shape-retaining, is a zone in which liquid-like fiowof the fiuid fibers occurs. In this zone the superticially coagulatedfiber has different properties than after it has gelled more completely,and the length of this liquid flow zone can usually be observedocularly. When the coagulating conditions of the present invention areemployed, the zone of liquid fiow extends at least 0.025 inch and notover ,0.2 inch from the spinneret for each foot per minute linear rateat which the fibers are withdrawn from the coagulating ba-th. Whencoagulating conditions are too abrupt, as when water is the coagulant,that zone extends less than 0.025 inch from the spinneret for each footper minute rate of withdrawal of fibers from the .coagulating bath. Aliquid zone length much over 8 times the stated minimum accompaniesinadequate coagulation. These factors show that the fiber formingmaterial must pass through the liquid flow zone in from 1A; to l secondafter leaving the orifice, and should be sufficiently gelled by thattime to be self-supporting.

The accompanying drawings illustrate various features of the invention,as follows:`

Fig. 1 is a liow diagram of a typical spinning opera-l tion useful withthe invention;

Fig. 2 is a typical curve showing the effect of spin-- neret size onspinning pressure;

Fig. 3 is a typical curve showing the maximum permissible stretchwithout rupture in coagulating baths of various concentrations;

Fig. 4 is a plot showing the effects of variations in temperature andconcentration in the coagulating bath on the character of the fibersspun in such baths; and,

Fig. 5 is a typical curve showing the amount of water required to beevaporated when recovering the saline constituents from coagulatingbaths of various concentrations.

As may be seen from the drawings, all other factors being kept constant,the lower the concentration of thc coagulating bath, the more difiicultit is to make good fibers economically. Thus, when water or weak brinesare used for coagulation, low temperatures must be used to get goodfibers (Fig. 4); little cold stretch is possible (Fig. 3) and hence,small orifices must be used to make fine fibers; small orifices requirehigh pressures (Fig. 2), and weak brines multiply the cost of saltrecovery (Fig. 5). However, at salt concentrations much over 47 percentin the coagulating bath (in the case of zinc chloride systems) Athere isso little coagulating tendency that the spun dope tends to dissolve ordisperse in the bath rather than to form a self-supporting fiber, andthe theoretical economies of operating at such high bath concentrationsare nullied. The most practical conditions as to spinneret size andcoagulant concentration are those found below and to the right, on Figs.2 and 5, of the points at which a line drawn tangent to the curves has aslope of 45, i. e., sp-inneret diameters of 0.015 inch or greater andzinc chloride concentrations in the bath of 25 to 47 percent.

l't has thus become apparent that the optimum conditions for makingfibers of good quality are those in which the coagulation is effectedslowly enough, and the first formed superficial gel is plastic enough,so that the fiber has time to be brought to the dimensions at which itwill leave the coagulating bath before it has coagulated completely, orhas formed a somewhat rigid sheath of coagulum. The desired slowcoagulation is accomplished by reducing the rate of diffusion of thesaline constituent from the fiber to the bath, through the use of astrong solution in water of the same saline material. This practiceslows the initial rate of coagulation on the surface of the fiber, andpermits the diffusion to proceed further into the interior before thesurface layers become so low in salt concentration as to beunstretchable under the low takeaway tension. The practice also producesa softer and more plastic gel in the surface layers of the fiber, andthese layers can undergo the necessary deformation duc to stretching ordue to volume changes during leaching without becoming cracked, strainedor disproportionately oriented.

When an aqueous saline solution of a fiber-forming polymer or copolymerof acrylonitrile is spun into a co agulating bath, diffusion starts atonce, and the interface between the fiber and the bath immediately comesto a concentration which does not vary greatly from the average of thetwo initial concentrations in the bath and in the spinning dope. Inorder to form a coherent fiber, it is necessary that this interface beat a low enough concentration so that a superficial gel is formed withina fraction of a second to give enough strength to the fiber so that thespun dope does not sink to the bottom of the bath, or dissolve in thebath, or develop a rough and irregular surface.

For each solution of any polymer or copolymer of acry- .lonitrile in anyaqueous saline medium, there is a minimum concentration of the salt orsalts below which the polymer will not remain dissolved at theprevailing temperature, and coagulation must occur in a bath which is atsuch a concentration that the salt concentration at the Spinning dopeaconcentratonPrecipitation concentration=tPreoipitationconcentration-.Upper limit .of coagulant concentration.

Thus, at C. the p-recipitation value for polyacryloni- -trile spun fromaqueous Yzinc 'chloride -solution is 'about 5 3 percent zinc chloride byweight, and a concentration below this value must be attainedimmediately at the liber surliace when Aan 'aqueous :zinc `chloridesolution of polyacrylonitrile lis Ispun into an aqueous coa'gulant at 15C. The precipitation value is slightly flower at higher :temperatures,:as is shown by the line :EF in Fig. 4v4. At 15 C., then, with anaqueous zinc chloride system, the most concentrated coagulant which canbe used is determined by the equation:

Spinning :dope concentration-53% =53 -upper limit of coagulantconcentration.

Assigning the conventional symbol X to the unknown upper 'limit ofcoagulant concentration, the foregoing equation may be simpliiied, asfollows;

Percentzsaline concentration tinspinnin'g dope -l- X;% 2

This maximum concentration 'of zinc chloride in the co- :agull'ant 'isshown, :for ra rrange of Y'ter'nperatures 'and for a spinning dopecontaining polyacryl-onitrile in -6.0 `percent aqueous zinc chloride, ascurve ASB 'in Fig. '4. It is apparent that the coagulrant vconcentrationmust "be 'reduced proportionately whenever ythe concentration of zincchloride is increased above 60 percent "in the spinning dope. It isconsidered undesirable vand irnpraot-ic'al to use much more salt 1in thespinning dope than is necessary to 'dissolve v'the polymer, andapplicants prefer to use zinc chloride solutions -o'f 58 'to 6l percentconcentration in the spinning dope.

`=It is `recognized that the line AfB of lFig. 4 ('or similar 'linesdetermined in `analogous manner for other systems) yd'etines a limi-tingbath :concentration tor vproducing quality iibers which, lafter standardiinishing operations have high toughness values and uniform orientation.For -ease in controlling the bath concentration, Tand to provide asafety-margin yforprotection against-the possible variations Yinconcentration of the bath across 'a multiple larnent tow ofsimultaneously extruded fibers, it is considered desirable to use `azcoa'gu-latin'g bath yconcentration to the left of curve AB (lFig. 4),since the ease of operation away from critical limiting lvalues morethan offsets small losses in toughness. yThe coagulation baths of the:present invention should :be at concentrations to the rig'ht vof curveCD, however, as th-ose to the `left Aof `CD 'rigidity 'the ,'gel sorapidly that poor tiibers are obtained, 4and so little liquid ow ispossible that the extruded product can be stretched not over 42.5 Vtimesin the bath, often much less than that, without breaking. For lreasonsgiven before, it should be possible to vary the speed at which the.tiber is Vtaken rfrom :the coagulating bath 'from 'the linear speed 'atwhich it is vextrud'ed'into the lbath to 4 tor more times .tha-t speedWithout breaking the tiber. it is not always necessary to withdraw aiber lat 4 or more times the linea-r rate of extrusion, but, to makegoodibers consistently `the coagulation rate -should :be such yas tomake possible the use of 4 to l, or greater rates of 'takew-ay.

IIt is noted that, at coagulation Itemperatures approaching C., the -ABcurve and .the CD curve of Fig. 4 appreach one another. This means that,[in the zone near the .BC segment of the curve, operating conditionsbecome more critical and there is less yfreed-om of choice of con-`dit-ions Liff good iibers are to be made. lIn 'the BC zone, diftusion'rates and coagulation rates .are much higher than at Alowertemperatures.

Unlike the coagulated wet-spun organic solutions of .acrylonitrilepolymers, the coagulated salt-spun vfibers behave like permeablemembranes, losing salt to the bath and absorbing wlater therefrom, andthe concentraition of salt in the coagulatin-g bath-controls 'the rateof this interchange and the Iquality of the bers. When coagulation iscontrolled as here described, the libershave vthe vproperty tot liquidil'ow for a short but significant time, and Venough reduction may bemade in Ithe diameter of the .freshly coagulated bers .to make it.possible to .defliver to the iinal wash baths and subsequent hotvstretching operations a fiber especially suited for the intended use.

The maximum cold stretch obtainable Without breaking the lgelled bers inthe `coagulating bath is shown in Fig. 3 tor operations lat 27 C., withvarious concentrations of coagulating baths, using a spinneret 0.015inch in diameter. vrl`he Aapproximate extent to which the curve isalected by changes in coagulation temperature is shown by ,dotted lines.The vfollowing table gives a relationship between the required amount ofcold stretch to produce a 3-denier liber, assuming in each case theWashed y-gelled fiber will be 'given a 10-to-1 1ro-t stretch, whenspinning a solution of polyacrylonitrile in 60 percent zinc chloridesolution through various sizes of spinnerets into a coagulating Vbathconsisting of 4l percent zinc chloride solution.

Orifice diameter: Required cold stretch 0.015 inch 10:1 0.0106 inch 5:10.0075 inch 2.5:1 0.00475 inch 1:1

The procedure of the invention provides a uniformly gelled liber by thetime the ber has been drawn down to the desired diameter and Washedessentially free of salt. The liber is then subjected to such typicalafter-treatment as stretching in steam `or hot water to effectorientation and vfurther reduction in diameter. It is found that theproperties of the so-'oriented bers, when dried, ,follow an orderlypattern dependent upon the concentration of the bath in which they wereyfirst coagulated rand up'on the iinal hot stretch ratio. The termstretch ratio employed herein refers to the additional length to which aiiber is stretched for each unit length of the fiber entring fthestretching zone. The following typical data illustrate the effects ofcoagulant concentration and inal hot stretch ratio on the ultimateproperties or iibers of polyocrylonitr-ile produced under otherwiseidentical conditions, vThe spinning dope was an 11 percent by weightsolution of polyacrylonitrile'in 60 percent aqueous zinc chloridesolution. It 'had a viscosity of 2000 poises and was spun at the rate of0.012 pound per orifice per hour through a 15'00-hole spinneret, :eachorifice being 0.015 inch in diameter, into the stated concentration ofaqueous zinc chloride coagulant at 20 C. All samples were given auniform cold stretch in the coagulating bath of 8:1, and, after beingwashed free of salt Without further stretching, were stretched theindicated amount in wet steam. When dried, the so-formed iib'ers had theproperties shown in the following table. rThe table shows a slight lossin tenacity with increasing coagulating bath concentration at a givensteam stretch ratio. However, when higher concentrations are used, it ispossible to steam stretch the fibers at a greater ratio, thus ultimatelyproducing a superior product. Por example, when fibers were spun into acoagu-lating bath vconcentration of 42.5% ZnClz, .it was not possible tosteam stretch without breaks vat over 12:1, but with .a coagulatingbathconcentration of 45.5% ZnClz, a steam stretch ratio of 16:1 wasachieved anda ber of 4.4 grams per denier tenacity and 29% elonga- .tionwasprodu-ced.

FIBER PROPERTIES vs. COAGULATING BATH CONCENTRATION AWD HOT STRETCHRATIOS Elongation, Percent Coagulant Conccn- Steam Stretch RatioTenacity, grams per denier Steam Stretch Ratio Denier Steam StretchRatio tration, Percent ZnGla 'iigol -ift" 'ftl' 'ai-ifi "sli Average. 4.1 3. 7 3. 2. 9

Notruf-In any single run, the variation in denier was of the order of 5percent.

l Noria-This concentration of coagulnnt not; usable under theseconditions, as thc bers break in thc eoagulating bath.

The following examples illustrate the practice of the invention:

Example l It was desired to malte 3-denier iibers, using a spinneret0.003 inch in diameter. The spinning dope was a 10.5 percent solution byweight of polyacrylonitrile in an aqueous brine consisting of 35.35percent zince chloride, 25.95 percent calcium chloride and 38.7 percentwater. The coagulant was at 5 C. and consisted of 22 percent zincchloride, 16 percent calcium chloride and 62 percent water. The zone ofliquid iiow outside the orifice was about l inch long. The coagulatediilaments rem'ained in the coagulating bath for l seconds and werewithdrawn from the bath at a linear rate 'of cabout 18 feet per minute,or about 4.5 times the linear extrusion rate. After further washing inmore dilute ZnClz-CaClz solution and in fresh water, they were stretched2-fold in wet steam, to about 10 times their original length. After aiinal water wash they were dried in an -air oven and had a denier of 3.0and a tenacity of 2.43 grams per denier, and were capable of 28 percentelongation.

By way of contrast, some 'of the same dope was spun through the samespinneret into water at C. The zone of liquid iiow was less than 0.1inch long. The so-coagulated fibers were removed from the bath at |aboutfeet per minute, which was the maximum rate possible without rupture,and was `only 2.5 times the linear rate of extrusion. After furtherwashing in water, they were essentially salt free, and were stretched asmuch as possible in wet steam, which was 1.7 times, to a total of about4.25 times their original length. When washed Iand dried, rthese bershad a denier of 5.5, a tenacity of 1.60 grams per denier and werecapable of 21 percent elongation.

Example 2 An l1 percent solution of p'olyaciylonitrile in 60 per centzinc chloride was spun through a 150G-hole spinneret having 0.015 inchoriices at the rate of 2 pounds of polymer per hour. ln order to drawdown the iibers enough inthe coagul'ating bath to make possible theproduction of l to 3 denier fibers after a normal 8-to-l hot stretchingoperation, it was found that the coagulating bath at C. must be at least4l percent zinc chloride and not over 45.5 percent zinc chloridesolution in water. At lower concentrations of zinc chloride in thecoafgulant, only coarse fibers could be made, and at higherconcentrations fibers could not be produced.

Example 3 A series of runs was made in which polyacrylonitrile solutionin aqueous saline media were spun at each of several temperatures intoeach of several concentrations of the same salt o1' salts in water. Theso-coagulated fibers were given as nearly as possible the sameaftertreatments, including the maximum permissible cold and hotstretching treatments without rupture. The dried fibers were tested todetermine their tenacities, in grams per denier, and their percentelongation under stress to the breaking point. A conventional, butarbitrary, toughness factor was determined for each fiber by means ofthe equation: Toughness=Vz tenacity x percent elongation. The resultsare tabulated below.

RELATIVE TOUGHNESS FACTOR vs. COAGULANT CONCENTRATION AND TEMPERATURETemperature of Coagulunt, C. Saline Content. of

lt is observed that, while many of the tested conditions have been shownherein not to be ideal, the fibers coagulated in fairly strong solutionsof the same salt system as that from which they are spun have a superiorcombination of tenacity and elongation values. Thus, fibers spun into a43 percent solution in Water of the salt have at least as great atoughness factor when coagulated at 25 C. as those coagulated in waterat 5 C., and, at any given temperature of coagulation the Order ofpreference is always the same.

The invention has been illustrated with respectto polyacrylonitrile, butis equally applicable to the liber forming copolymers of acrylonitrile,and is especially advantageous with those copolymers containing percentor more of acrylonitrile in the polymer molecule, as these most closelyresemble the homopolymer in their behavior.

This application is a continuation-impart of our c0- pending'application Serial No. 228,755, tiled May 28, 1951 now abandoned.

We claim:

1. The method which comprises spinning an aqueous inorganic salinesolution of a fiber-forming polymer containing at least 85 percent ofacrylonitrile in the polymer molecule through a spinneret into acoagulating bath which is at a temperature not in excess of 30 C. andwhich consists essentially of a solution in water of thc same salinematerial as is in the solution being spun, the saline concentration ofthe coagulating bath being in the range from 25 percent to a higherconcentration of X percent and having such a value in said range thatthe freshly extruded fiber exhibits a zone of liquid iiow extending fromthe spinneret orilice a distance equal to from 0.025 to 0.2 inch foreach foot per minute linear rate at which the liber is withdrawncontinuously from the coagulating bath, thereby permitting the rate ofwithdrawal of fiber from the bath without breaking to be at least 4times the linear rate at which the polymer solution is spun into thecoagulating bath; Washing the so-coagnlated fiber to free it of salt,and hot stretching it; the limiting concentration X percent beingdetermined from the equation:

Percent saline concentration of spinning dopc-l-X] the length of saidzone of liquid flow, and the permissible rate of withdrawal of liberfrom the coagulating bath, both varying in the same direction asvariations 9 10 in the coagulating bath concentration in said range frommer is polyacrylonitrile, the saline constituent of the 25 percent to Xpercent. solution being spun is zinc chloride, and the coagulating 2.The method claimed in claim 1, wherein the polybath is a solution ofzinc chloride in water. mer is polyacrylonitrile. Y

3. The method claimed in claim 1, wherein the saline 5 References Citedin the me 0f this pant constituent of both the spinning dope and thecoagulat- UNITED STATES PATENTS ing bath is predominantly zinc chloride.

4. The method claimed in claim 1, wherein the saline 2,025,730 Dickie etal Dec. 31, 1935 constituent in the spinning dope is essentially zincchlo- 2,140,921 Rein Dec. 20, 1938 ride and the coagulating bath is asolution of zinc 10 2,558,630 Cresswell July 3, 1951 chloride in water.2,577,763 Hoxie Dec. 11, 1951 5. The method claimed in claim 1, whereinthe poly-

1. THE METHOD WHICH COMPRISES SPINNING AN AQUEOUS INORGANIC SALINESOLUTION OF A FIBER-FORMING POLYMER CONTAINING AT LEAST 85 PERCENT OFACRYLONITRILE IN THE POLYMER MOLECULE THROUGH A SPINNERET INTO ACOAGULATING BATH WHICH IS AT A TEMPERATURE NOT IN EXCESS OF 30*C. ANDWHICH CONSISTS ESSENTIALLY OF A SOLUTION IN WATER OF THE SAME SALINEMATERIAL AS IS IN TH ESOLUTION BEING SPUN, THE SALINE CONCENTRATION OFTHE COAGULATING BATH BEING IN THE RANGE FROM 25 PERCENT TO A HIGHERCONCENTRATION OF "X PERCENT" AND HAVING SUCH A VALUE IN SAID RANGE THATTHE FRESHLY EXTRUDED FIBER EXHIBITS A ZONE OF LIQUID FLOW EXTENDING FROMTHE SPINNERET ORIFICE A DISTANCE EQUAL TO FROM 0.025 TO 0.2 INCH FOREACH FOOT PER MINUTE LINERAR RATE AT WHICH THE FIBER IS WITHDRAWNCONTINUOUSLY FROM THE COAGULATING BATH, THEREBY PERMITTING THE RATE OFWITHDRAWAL OF FIBER FROM THE BATH WITHOUT BREAKING TO BE AT LEAST 4TIMES THE LINEAR RATE AT WHICH THE POLYMER SOLUTION IS SPUM INTO THECOAGULATING BATH; WASHING THE SO-COAGULATED FIBER TO FREE IT OF SALT,AND HOT STRETCHING IT; THE LIMITING CONCENTRATION "X PERCENT" BEINGDETERMINED FROM THE EQUATION: